Railway track circuit



Nov. 5, 1957 F. B. HITCHCOCK 2,812,425

RAILWAY TRACK cmcurr Filed Oct. 4, 1955 FIG. I;

A R 0 CR VR I I 8\ 9 I- 4 5 v H I f) I 7 6| I SIGNAL i CONTROL CIRCUIT FIG. 2.

5O RELAY CURRENT (ma) TIME INVENTOR.

United States Patent Ofiice RAILWAY TRACK CIRCUIT Forest B. Hitchcock, Rochester, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Application October 4, 1955, Serial No. 538,412

6 Claims. (Cl. 246-41) This invention relates to railway track circuits of the type used in controlling signaling devices. More particularly, this invention relates to means for compensating for changes in track ballast resistance caused by weather and/ or other conditions, thereby maintaining the shunting sensitivities of track relays at a more uniform level.

The well-known normally closed track circuit is a series circuit including a source of energy, the track rails, a track relay and a variable resistor for adjusting interrail potential. The source of energy is usually connected to the rails at one end of the track section; and the track relay is connected to the rails at the other end of the track section. When a train occupies the rails of the track circuit, the wheels and axles of the train provide low resistance paths for current to how between the rails, which paths are in parallel with the track relay winding. The resulting increase in current in the series resistance of the track circuit results in the lowering of the interrail voltage at the relay end of the track section. Such voltage drops cause the current in the relay winding to drop below the value required by the relay for holding its armature in its attracted, or picked-up, position.

Track ballast provides paths of varying resistance for current to flow between the rails, which paths are in parallel with the track relay winding. Ballast resistance is a minimum when the ballast is wet and is a maximum when the ballast is dry. Therefore, voltage applied to the track circuit must be high enough to permit the track relay to pick up its armature when ballast resistance is low. However, an applied voltage adjusted for conditions of low ballast resistance must not be high enough to make the track relay unresponsive to train shunts. A general solution to this problem is to provide means for detecting conditions of high and low ballast resistance as reflected by the interrail voltage at the track relay end of the track circuit and adjust the track circuit accordingly. For example, the detection means then inserts or removes a resistance in series with the track relay in the track circuit. Thus, the voltage applied to the track relay is determined by conditions of ballast resistance.

The present invention proposes the use of a mechanically interlocked relay which comprises two independent electromagnetic structures, each electromagnetic structure having an associated armature; and the armatures are interlocked mechanically in such a manner that when one armature is in its dropped-away position it prevents the other armature from attaining its dropped-away position. It is proposed that one of the electromagnetic structures be used as a track relay and that the other electromagnetic structure be used as a relay for detecting conditions of high ballast resistance. For simplicity the two electromagnetic structures will be hereafter referred to as the track relay and the detection relay.

In the present invention, the windings of the track relay and the detection relay are connected in series in 2,812,425 Patented Nov. 5, 1 957 the track circuit. The operating characteristics of the track relay are assumed to be such that the relay is suitably responsive to applications and removals of train shunts under conditions of low or normal ballast resistance. The operating characteristics of the detection relay, however, are assumed to be such that it is capable of picking up its armature only when ballast resistance rises above a predetermined level. The detection relay is further assumed to be slow-acting in releasing its armature. Thus, when a train shunt occurs, both relays tend to release their armatures; but the track relay armature is actually released first to eliect an interlock to hold the armature of the detector relay mechanically picked up regardless of the degree of energization of the detector relay.

Also, the armature-operated contacts of the detection relay insert resistance in series with both relays when the detection relay picks up its armature as the result of a critical increase in ballast resistance. The increased series resistance is incapable of lowering the current in the winding of the detection relay below the value at which the relay releases its armature. The series resistance does, however, reduce the current in the track relay so that the relay is made more responsive to train shunts. When a shunt occurs across the track rails, the track relay releases its armature, thereby locking the armature of the detection relay in a manner such that the armature of the detection relay cannot drop away to the extent that back contacts of the relay close. The series resistance is thereby retained in the relay circuits. The detection relay can release its armature to its full droppedaway position whenever the ballast resistance decreases to the point where the current in the detection relay winding falls below the drop-away value for the relay.

' This does not drop the track relay because the dropaway current value of the track relay is slightly below the drop-away value of the detection relay.

It is further proposed, that the track relay be used as the primary relay in a primary-secondary track relay organization. A secondary repeater relay, having slowacting characteristics in picking up its armature, is provided to repeat the primary track relay. When the primary relay releases its armature, the winding of the secondary relay is disconnected from a local source of energy. When the primary relay attracts its armature, the secondary relay is energized. Contacts of the secondary relay are used to vary the number of turns of the primary relay winding connected into the track circuit.

Whenever a train shunt exists the entire Winding of the primary relay is connected into the track circuit. When the train shunt is removed the current rises in the primary relay winding causing the picking up of the primary r lay armature, which causes the secondary relay to become energized; and its contacts cut out part of the winding of the primary relay. The reduction in the number of effective turns in the winding of the primary relay makes the relay more responsive to train shunts. The secondary relay is usually made slow-acting in picking up its armature so that it is not operated if the primary track relay should momentarily pick up its armature in response to momentary losses of shunt caused by high contact resistance between wheels and rails.

In view of the above, one of the objects of this invention is to provide ballast variation compensating means which is organized on the closed-circuit principle, sometimes termed the fail-safe principle.

Another object of the present invention is to provide a ballast compensating means which is simple in structure and which employs relatively simple mechanical interlocking means for interconnecting the regular track relay and the ballast compensator.

A further object of the present invention is to provide the detector relay of the ballast compensating portion of the structure with operating characteristics which have particular relationships with respect to the operating characteristics of the track relay.

Further objects, purposes and characteristic features of the present invention will be in part pointed out as the description of the invention progresses, and will be in part obvious from the accompanying drawings.

In the accompanying drawings:

Fig. 1 shows diagrammatically a track circuit organization embodying this invention;

Fig. 2 shows graphically the current flowing under various conditions in the windings of the track and detector relays which are mechanically interlocked in accordance with this invention; and

Fig. 3 shows an enlarged diagram of the manner in which the armatures of the track relay and the detection relay are interlocked mechanically to meet the operational requirements of the present invention.

For the purpose of simplifying the illustration and facilitating in the explanation, the various parts and .cir-

cuits constituting the embodiment of the invention have been shown diagrammatically and certain conventional illustrations have been employed, the drawings having been made more with the purpose of making it easy to understand the principles and mode of operation, than with the idea of illustrating the specific construction and arrangement of parts that would be employed in practice. Thus, the various relays and their contacts are illustrated in a conventional manner, and symbols are used to indicate connections to the terminals of batteries, or other sources of electric current, instead of showing all of the wiring connections to these terminals.

The symbols and are employed to indicate the positive and negative terminals, respectively, of suitable batteries, or other sources of direct current; and the circuits with which these symbols are used, always a have current flowing in the same direction. a

In Fig. 1, reference characters 1 and 2 designate the rails of a section of railway track. The rails 1 and 2 are parts of a track circuit and are insulated from the rails of adjoining track sections by insulated rail joints 3.

At one end of the track section, a battery B and a variable resistor VR are connected in series to the righthand ends of the rails 1 and 2. At the other end of the track section the windings of a trackrelay T and a detection relay D are connected to the rails 1 and 2 in a manner to be described. Relays T and D arecomponent parts of an interlocking relay and are independently'operable. Howeventhe armatures of therespective relays are mechanically interlocked in a manner such that whenever both armatures are picked up a subsequent dropping away of the armature of relay T prevents the armature of relay D from assuming its dropped-away position.

In Figs. 1 and 3, the reference characters A1 and A2 designate the armatures of relays T and D, respectively. The armature A1 is assumed to be pivotally supported near its left extremity by a pin '8, while the armature A2 is assumed to be supported pivotally near its right extremity by a pin 9.

A mechanical interlock between the armatures A1 and A2 can be of any of a number of types. For purposes of illustration, one type of interlocking means is shown to comprise an operating arm 4, a latch arm '5 and a spring 15. The operating arm 4 is pivotally supported at a point 6 and so located that it always is positioned beneath the armature A1. An L-shaped portion of the operating arm 4 makes contact with the latch arm which is pivotally supported at a point .7. A latching projection SA on the latch arm 5 is provided for obstructing movements by the armature A2 as will be described. The spring represents one means forapplying a bias ing force on the latch arm 4, causing the latcharm to assume a particular normal position, as shown in Fig. 1.

The relative positions of the armatures and the inter locking members under conditions of low ballast resistance and the absence of train shunts are shown in Fig. l. The armature A1 is in its picked-up position while the armature A2 is in its dropped-away position; and broken lines indicate the-alternate operating positions of each armature. The spring 15 positions the latch arm 5 as shown; and the spring acts through the latch arm 5 to position the operating arm 4 accordingly. As can be readily seen, the latching projection 5A is removed from the path of movement of the armature A2; and this armature is free to move in acordance with the degree of energization of the relay D caused by changes in ballast resistance. Under conditions of high ballast resistance, therefore, the armature A2 is operable ,to its picked-up position.

Whenever a train shunt occurs both relays T and D release their armatures. The armature A1 of relay T, in dropping away, causes the operating arm 4- to be rotated counter-clockwise; and the L-shaped portion of the operating arm 4 moves the latch arm 5 to the right (clockwise), thereby placing the latching projection 5A in the path of motion of the armature A2.

Relay D is assumed to be slow-acting in releasing its armature. Therefore, when a train shunt occurs the armature A1 of relay T drops away before the armature A2 of relay D. Thus, under conditions of high ballast resistance the arrival of a train shunt causes the arrnatures and interlocking members to assume positions as shown in Fig. 3. The armature A2 rests on the latching projection 5A, and will be held in this position until relay T is again able to pick up armature A1. It is assumed that under the conditions shown in Fig. 3 the armature A2 is prevented from dropping away to any position which would result in the closing of armature-operated back contacts, such as 11 in Pig. 1, of relay D.

When the removal .of a train shunt causes relay T to pick up armature Al, the operating arm 4 is free to rotate. The spring 15 pulls the latch arm 5 to the left (counter-clockwise), and the operating arm 4 is rotated clockwise because of the force applied to its L-shaped portion by the latch arm 5. The latching projection 5A is pulled from underthe armature A2, thereby clearing the path of motion of armature A2.

It is evident from the schematic physical arrangement of parts'in Figs. 1 and 3 that the spring 15 is able to exert vertically no reactive force against the armature A1 through the members' i and 5. The structure and arrangement of the various members in conjunction with the operating characteristics of the relays (especially relay T) can be so related that the ability of the track relay T to pick up its armature under any operating conditions is not afiected'by the mechanical interlock.

The arrangement of parts and the mechanical relationships must also be such that the spring 15-is capable of pulling the latch arm 5 to the left even though the armature A2 may be supported by the latchingprojection 5A. In other words, upon the removal of a train shunt it is possible that the relay D cannot be energized sufiiciently to pick up armature A2, such conditions being caused by the lowering of ballast resistance during the shunting period. Refrigerator cars, for example, included in a train can deposit-brine solutions which decrease the ballast resistance. Thus, when the armature A2 continues to rest on the latching projection 5A, the spring 15 must be able to pull the latching projection 5A from under the armature A2 so that-this armature is permitted to drop away. Actually, the supporting surface of the latching projection 5A is assumed to be curved (the pivot point 7 being the center of curvature) so that the latch arm 5 is not required to lift the armature A2'as the arm 5 moves to the left. Also, it is assumed thatsuflicient tolerance is provided to permit relatively free sliding between the contact surfaces of the latching projection 5A and the armature A2. g

As indicated in Fig. 1 the operating arm 4 does not maintain contact with the armature A1 when that armature is fully picked up. In this manner, the spring 15 is unable to exert even a small reactive force against the armature A1. Thus, no initial upward biasing force is applied against downward movements of the armature A1 when a train shunt occurs. Actually, the arrangement of parts can be such that the armature A1 moves far enough to open the front contacts of the relay T before coming in contact with the operating arm 4. A suitable stop 16 is assumed to be employed for limiting the travel of the operating arm 4, thereby fixing a minimum spacing between the armature A1 and the operating arm 4 under the conditions shown in Fig. 1.

Since a higher value of current is required by relay D to pick up armature A2 than is required by relay T to pick up armature A1, it is impossible for a situation to arise wherein the armature A2 attempts to force its way past the latching projection 5A in an upward direction. Thus, whenever both armatures are dropped away and the latch arm 5 is positioned for locking, an increase in current through the relay windings sufficient to cause the picking up of relay D would be more than sufficient to cause the picking up of relay T. A reactive force tending to pick up the armature A1 cannot, therefore, ever be applied through the interlocking structure by the armature A2.

The preceding description of a mechanical interlock has been given to show means by which relay armatures can be interlocked to provide the mode of operation required for any interlocked relays used in the present invention.

When the track circuit is initially placed into service, the circuit can be traced from the positive terminal of battery B through the variable resistor VR, rail 1, back contact ill of relay S, the entire winding of relay T, the windin of relay D, back contact 11 of relay D which shunts a compensating resistor CR, and rail 2 to the negative terminal of battery B. The variable resistor VR is adjusted so that the interrail voltage at the relay end of the track circuit is sufficient to cause relay T to pick up its armature. Since the interrail voltage at the relay end of the track circuit is dependent upon the resistance of the track ballast, which provides shunt paths between the rails, the resistor VR is adjusted to anticipate the worst probable conditions of low ballast resistance. Relay T then picks up its armature, closing its front contact 12 to ener lze relay 8. Contact ll of relay S crosses over from its back to its front contact point; and it is assumed that contact is of the make-before-break type. In other words, the movable contact portion of contact is always in contact with one of the stationary contact points. Relay T is, therefore, not deenergized during the crossover of contact it. of relay S. The back contact point of contact ill of relay S is connected to one extremity or" the winding of relay T, while the front contact point of contact 10 of relay S is connected to the intermediate tap of the winding of relay T through a limiting resistor R. Thus, contact 1% of relay S removes the entire Winding of relay T from the track circuit and places a portion of the winding into the circuit in series with resistor R. The ohmic resistance of the limiting resistor R is equal to the resistance of the portion of the winding of relay T which was removed from the track circuit. Thus, the number of effective turns in the winding of relay T is reduced, while the current in the windng is held at the same level. The reduction in the numser of effective turns makes the relay more responsive to voltage drops which result from train shunts. Obviously, the reduction in effective turns is limited by the ampereturns required to retain the relay armature A1 in its picked-up position.

The detection relay D is provided to operate whenever ballast resistance rises above or falls below predetermined levels. Since an increase in ballast resistance produces an increase in interrail voltage at the relay end of the track circuit, the responsiveness of relay T to train shunts is reduced. The upper limit of the voltage which can be allowed without critically affecting the response of relay T determines the operating characteristics of relay D. In other words, when the critical interrail voltage is exceeded, relay D picks up its armature. As a result, back contact 11 of relay D opens, removing the shunt path around resistor CR. Resistor CR is effectively placed in series with the windings of relays T and D, thereby reducing the current in the windings. The resistance of resistor CR is such that the relay current is reduced to a value which is higher than the value at which relay D releases its armature and which is within the range of current values encountered when ballast resistance is low. Therefore, the insertion of resistor CR compensates for the increase in interrail voltage at the relay end of the track circuit by reducing the voltage across and the current in the winding of relay T; thus the current supplied to the windings of relay T will not exceed the maximum operating value characteristic of that particular relay.

A subsequent decrease in ballast resistance causes the interrail voltage at the relay end of the track section to decrease. When the voltage across relay D reaches the drop-away value, relay D releases its armature. The subsequent closing of back contact 11 of relay D shunts the resistor CR. The current in the windings of relays T and D increases, but does not reach the magnitude necessary to cause relay D to pick up its armature.

Thus, relay D responds to voltage changes resulting from appreciable changes in ballast resistance. Relay D operates to maintain a particular range of voltage across the winding of relay T by causing the compensating resistor CR to be either inserted into or shunted out of the track circuit.

Assume now that a train enters the track section at a time when ballast resistance is high and relay D has picked up its armature, thereby causing its back contact 11 to open and effectively insert resistor CR into the track circuit. The low resistance shunt produced by the wheels and axles of the train sharply lowers the interrail voltage at the relay end of the track section. The current in the windings of relays T and D drops below the values at which the respective relays release their armatures. As previously described, the armatures of relays T and D are interlocked mechanically so that when the armature A1 of relay T assumes its dropped-away position, the armature A2 of relay D is prevented from assuming its dropped-away position. In order to obtain the required results from the mechanical interlocking feature, relay D is made slow-acting in releasing its armature to the extent that the armature of relay T assumes its dropped-away position before the armature of relay D is released. Since under these conditions the armature of relay D cannot attain its dropped-away position, back contact 11 of relay D cannot close. Therefore, the compensating resistor CR. remains effectively connected into the track circuit. If contact 11 of relay D were allowed to close and shunt out resistor CR, the voltage across the relay windings would be higher. Thus, the shunting sensitivity of the track circuit would be made poorer in that the ability of the relay T to pick up its armature would be greater; and the incidence of high resistance shunts could cause more easily the unsafe picking up of the armature of relay T. When the train shunt is removed, relays T and D again pick up their armatures.

Whenever relay T releases its armature, its front contact 12 opens to deenergize the secondary relay S. Back contact it) of relay S closes; and front contact 10 of relay S then opens. The entire winding of relay T is then connected into the track circuit. Should either a high resistance train shunt or a momentary loss of shunt occur at this time, relay T attracts its armature. Under severe conditions, front contact 12 of relay T might close the pick-up circuit for relay S. Relay S, however, is.

made slow-acting in picking up its armature and does not reflect momentary poor shunting conditions. Contacts of the secondary relay S, such as contact 13, are utilized in various control circuits for signaling devices.

It can be seen that contact 1%) of relay S is of the make-before-break type to preclude the deenergization of relay T at times when contact 16 changes position. For example, when relay T is shunted, resulting in the deenergization of relay S, a subsequent removal of the shunt causes relay T to pick up its armature. When contact 12 of relay T closes, relay S is energized. The resulting crossing over of contact 10 of relay S from the back contact point to the front contact point would cause the momentary deenergization of relay T unless a make-before-break feature is provided.

In Fig. 2, the values of current flowing in the windings of relays T and D under various conditions are plotted against time. Representative values of current are measured along the vertical axis, while arbitrary time intervals are measured along the horizontal axis. Various broken lines indicate the pick-up and drop-away values of current for the relays T and D. Lines TD and TP represent the respective current values at which relay T drops away and picks up its armature. Similarly, lines DD and DP represent the respective drop-away and pickup values of current for relay D. The various particular values of current shown point out comparative differences between the values of current which effect operations of the two relays. it can be noted that the value of relay current required by relay D for picking up its armature is less than the value shown by the line DP whenever the armature A2 of relay D is not permitted by the mechanical interlocking means to assume its fully droppedaway position. Since any intermediate value of pick-up current for relay D would be higher than the drop-away current value DD for the relay, the effect produced by the mechanical interlocking is not important. In other words, if the mechanical interlocking means is to be efiective at all, relay D must have its armature picked up; and when the armature of relay D is once picked up it is not released until the relay current falls below the drop-away current value for the relay D. Since any value of pick-up current must be higher than the dropaway value, it makes little difference how the pick-up value for relay D varies when the mechanical interlocking is in effect. No intermediate value of pick-up current is indicated for relay D in Fig. 2 since the particular mechanical interlocking structure shown in Fig. 3 does not permit the armature A2 of relay D to drop away at all under conditions when the interlocking means is effective.

Assuming that no train shunts are present in the track circuit and that conditions of low ballast resistance exist, but that ballast resistance is increasing, the current is shown rising between points a and b. The current level is above the pick-up value for relay T, but is below the pick-up value for relay D. Between points I; and c the current remains constant, indicating that the ballast resistance has assumed a relatively constant value.

Assuming now that a train enters the track section, the shunt produced by the Wheels and axles of the train causes the relay current to drop sharply as indicated between points and e; and at point d the drop-away value of current for relay T is reached. Relay T, therefore, releases its armature. The continued presence of the train shunt causes the current to remain at a steady value as indicated between the points 2 and 7.

When the train leaves the track circuit, the relay current rises because of the removal of the train shunt. Between points 7 and i, the current is shown to rise as a result of a combination of conditions under which a train shunt is removed and ballast resistance increases. At point g, the current level rises to the pick-up value for relay T; and relay T picks up its armature. At point h,

the current level reaches the pick-up value for relay D, causing that relay to pick up its armature. When relay D picks up its armature, the compensating resistor CR is effectively inserted into the track circuit as previously described, resulting in a decrease in relay current as indicated between points i and j. It should be pointed out that the current is assumed to reach a value (point 1) higher than the pick-up value for relay D before being reduced by the insertion of the compensating resistor CR. Such would be the case under the conditions described, wherein the removal of the train shunt results in a comparatively sharp rise in current. A rise in current caused by increasing ballast resistance alone, on the other hand, would be appreciably less sharp; and the value of current would not tend to exceed the pick-up value for relay D before being reduced by the insertion of the compensating resistor CR.

Assuming that conditions of high ballast resistance continue, the relay current remains constant as indicated between points 1' and k.

If a train shunt occurs at this time, relay current decreases as indicated between points and 11. At point I, the drop-away value of current for relay D is reached; but since relay D is slow-acting, it does not release its armature until after relay T releases its armature when the relay current falls below the drop-away value for relay T at point m. Thus, as previously described, the armature of relay D is prevented from reaching its dropped-away position because of the mechanical interlock between its armature and the armature of relay T. The compensating resistor CR is retained in the track circuit; and relay current remains constant for the duration of the train shunt as indicated between points 12 and 0.

When the train shunt is removed, relay current rises as shown between points 0 and q. At point p, the current level reaches the value required by relay T for picking up its armature. When relay T picks up its armature, the secondary relay S is energized as previously described.

Assume now that ballast resistance decreases because of weather or other conditions. Relay current decreases as indicated between points q and 1'. At point r, the dropaway value of current for relay D is reached; and relay D releases its armature causing the effective removal of the compensating resistor CR from the track circuit. Relay current then increases as indicated between points r and s. Upon reaching the level indicated at points, the current remains constant until other conditions similar to those described occur.

It is evident from the preceding descriptions that the track circuit in the present invention has high integrity of operation in that the shunting sensitivity of the track circuit is maintained at a relatively constant level. By providing a track circuit arrangement of the primarysecondary type, circuit operation is made independent of momentary occurrences of high resistance shunts, or losses of shunt. Furthermore, the provision of a mechanically interlocked relay is an efficient means for compensating for irregularities in circuit operations caused by changes in track ballast resistance. The use of a mechanically interlocked relay as an effective means for compensating for changes in ballast resistance is not confined to track circuits of the primary-secondary type as shown, but is applicable to other types of track circuits as well.

Having described a railway track circuit as one specific embodiment of the present invention, it is desired to be understood that this form is selected to facilitate in the disclosure of the invention rather than to limit the number of forms which it may assume, and it is to be further understood that various modifications, adaptations and alterations may be applied to the specific form shown to meet the requirements of practice, without in any manner departing from the spirit or scope of the present invention.

What I claim is:

1. In a track circuit for detecting the presence of trains in a railway track section, a source of energy connected to the track rails at one end of the track section, a track relay, a detector relay requiring relatively high energizing potential as compared to said track relay, a compensating resistor, circuit means for connecting said track relay and said detector relay and said compensating resistor in series across the track rails at the other end of the track secion, said circuit means including a back contact of said detector relay in multiple with said compensating resistor, and mechanical interlocking means preventing the dropping away of said detector relay when said track relay is in its dropped-away position.

2. In a track circuit for detecting the presence of trains in a railway track section, a source of energy connected to the rails at one end of the track section, a track relay and a relatively slow-acting detection relay, means for mechanically interlocking the armatures of said relays in a manner such that whenever the armature of said track relay assumes its dropped-away position while the armature of said detection relay assumes its picked-up posi tion the armature of said detection relay is prevented from assuming a dropped-away position wherein armatureoperated back contacts of said detection relay close, a compensating resistor, circuit means for connecting said compensating resistor in series with the windings of said track and detection relays and for connecting said compensating resistor and said relay windings to the rails at the other end of the track section, and means for shunting said compensating resistor whenever the armature of said detection relay assumes its fully dropped-away position.

3. In a track circuit associated with a railway track section, a source of energy connected to the rails of the track section at one end of the track section, a primary track relay having an intermediate tap provided in its Winding, a limiting resistor connected to the intermediate tap in the winding of said primary track relay, a detection relay, said detection relay being slow-acting in comparison to said primary track relay, means for mechanically interlocking the armatures of said primary track and detection relays in a manner such that whenever the armature of said primary track relay assumes its droppedaway position while the armature of said detection relay assumes its picked-up position the armature of said detection relay is prevented from assuming a dropped away position wherein armature-operated back contacts of said detection relay are closed, a secondary track relay, said secondary track relay being slow-acting in picking up its armature, pick-up circuit means including a front contact of said primary track relay for energizing said secondary track relay, a compensating resistor, circuit means including front and back contacts of said secondary track relay for connecting the windings of said primary track relay and said detection relay and said compensating resistor in series and to the rails at the other end of the track section, the back contact of said secondary track relay being connected to one extremity of the winding of said primary track relay and the front contact of said secondary track relay being connected to said limiting resistor, and circuit means including a back contact of said detection relay for shunting said compensating resistor.

4. In combination with a section of railroad track having its ends insulated from adjoining sections, a source of energy and a limiting resistor connected across the track rails at one end of said section, a track relay, a detector relay, a compensating resistor and circuit means connecting said track relay, said detector relay, and said compensating resistor in series across the track rails at the other end of said section, mechanical means for directly interlocking the armatures of said track relay and said detector relay thereby preventing said detector relay when once energized from dropping away when said track relay is in its deenergized position, said track relay adjusted so as to be responsive to train shunts entering onto said section when the value of the ballast resistance is at or below the normal value of ballast resistance for said section, a back contact of said detector relay directly controlling insertion or removal of said compensating resistor in said circuit means in accordance with the value of said ballast resistance of said section whereby said detector relay is adjusted so as to cooperate with said track relay to render said track relay also responsive to said train shunts when the value of ballast resistance exceeds the normal value of the ballast resistance for said section.

5. In combination with a section of railroad track having its ends insulated from adjoining sections, a source of energy and a limiting resistor connected across the track rails at one end of said section, two relays having their windings connected in series and their armatures mechanically interlocked, a resistor connected in parallel with a back contact of one of said two relays, circuit means including the parallel combination of said resistor and said back contact of said one relay for connecting the windings of said two relays across the track rails at the other end of said section, said one relay having a higher pick-up value than the other of said two relays whereby said one relay may insert or remove said resistor controlled by its back contact in accordance with a change in ballast resistance along said track section.

6. In combination with a section of railroad track having its ends insulated from adjoining sections, a source of energy and a limiting resistor connected across the track rails at one end of said section, a primary track relay having an intermediate tap provided in its winding, a secondary relay and a detector relay each having slow acting characteristics, a compensating resistor, and circuit means including contacts of said secondary relay for connecting said primary track relay, said secondary relay, said detector relay and said compensating resistor in series across the track rails at the other end of said section, mechanical means for directly interlocking the armatures of said primary track relay and said detector relay thereby preventing said detector relay when once energized from dropping away when said primary track relay is in its deenergized position, said primary track relay adjusted so as to be responsive to train shunts entering onto said section when the value of the ballast resistance is at or below the normal value of ballast resistance for said section, a back contact of said detector relay directly controlling insertion or removal of said compensating resistor in said circuit means in accordance with the value of said ballast resistance of said section whereby said detector relay is adjusted so as to cooperate with said track relay to render said track relay also responsive to said train shunts when the value of ballast resistance exceeds the normal value of the ballast resistance for said section, pick-up circuit means for energizing said secondary track relay whenever the armature of said primary track relay assumes its picked up position, said contacts of said secondary track relay being connected to one extremity and to said intermediate tap of the winding of said primary track relay in such a manner that whenever the armature of said secondary track relay assumes its dropped away position all of the turns in the windings of said primary track relay are effective and whenever the armature of said secondary track relay assumes its picked up position only part of the turns in the winding of said primary track relay are effective, and circuit means for shunting said compensating resistor whenever the armature of said detection relay assumes its fully dropped away position.

References Cited in the file of this patent UNITED STATES PATENTS 2,026,497 Hitchcock Dec. 31, 1935 2,135,497 Field Nov. 8, 1938 2,135,499 Field Nov. 8, 1938 2,135,548 Willing Nov. 8, 1938 

